1. Field of the Invention
The present invention relates to optical intensity modulation and in particular to the use of Surface Plasmon-Polaritons (SPPs) in an optical modulator system.
Optical intensity modulators are an important element for integrated optoelectronics. Surface Plasmon-Polaritons (SPPs) are attractive candidates for this use due to their potential to produce high-speed compact devices. SPPs are surface electromagnetic waves that are trapped at the metal-dielectric interface due to their interaction with valence electrons of the conductor. Recent demonstrations of light guiding on closely spaced metal nano-particles have created interest for nano-photonic devices that will merge photonic and electronic functionalities at nano-scale dimensions. Optical modulators by means of SPPs have been demonstrated by modifying the refractive index of the dielectric medium. However, the fact that electro-optic effects tend to be weak in semiconductors while large electro-optic effects are obtainable only in slower materials, such as liquid crystals, hinders the modulation speed of such proposed devices. The present invention however, improves modulation efficiency (or increases modulation speed) of an optical modulator utilizing Surface Plasmon-Polaritons (SPPs).
Approaches to surface plasmon optical modulation fall into three categories. One category is the “Refractive Index Approach,” whereby a high electro-optic dielectric layer is sandwiched between two metal films and wherein modification of the refractive index of the dielectric material results in changes in reflectivity and thus modulates the incident beam.
One implementation of the “Refractive Index Approach,” is discussed in the several U.S. Pat. Nos. 4,432,614, 5,067,788, 5,570,139, 6,034,809 and 6,611,367. This approach relies on the surface plasmon resonant angle θ for a single incident wavelength being a function of dielectric constant ∈d. According to this approach, a small 0.1% change of refractive index can result in a change in resonant frequency over 10%.
For a single incident wavelength, the optical reflectivity reaches the minimum at resonant angle θ and increases as an approximate Lorentz function with the angle θ. For a single incident wavelength at a fixed incident angle, a change of dielectric constant ∈d through the applied bias changes the optical reflectance and thus modulates the optical intensity. This approach uses capacitance as a means to modulate the optical intensity.
Another approach includes the use of a prism whereby the prism performs the function of phase matching so that surface plasmons can be excited by the incident light, as discussed in U.S. Pat. Nos. 4,432,614, 5,570,139 and 6,611,367. Other variations include using gratings and waveguides (as discussed in U.S. Pat. Nos. 5,067,788 and 6,034,809). An applied voltage across a relatively thick dielectric material (e.g., liquid crystal) changes its dielectric constant ∈d and leads to light modulation. The thick dielectric material requires a high dielectric constant ∈d for this principle to be applied. This approach also uses an applied bias to change the refractive index, which results in changing the resonant frequency or resonant angle θ.
Still another implementation of the “Refractive Index Approach,” is the approach which includes improving the modulation efficiency by selecting materials of a large electro-optic constant. However, this approach limits the modulation speed as a large electro-optic effect is generally obtainable only in slower reacting materials, such as liquid crystals.
Still another implementation of the “Refractive Index Approach,” is discussed in U.S. Pat. No. 6,611,367. The principle in this approach includes the use of exotic materials that may not be able to be integrated with silicon based fabrication processes.
A second category of surface plasmon optical modulation is the “Optical Absorption Approach,” where a semiconductor quantum well is formed immediately adjacent to a metal film, light is coupled to the surface plasmon polariton mode and then the absorption of the quantum well is altered by the quantum confined optical absorption region by a quantum Stark shift of the semiconductor.
An implementation of such as an approach is taken in U.S. Pat. No. 4,915,482. The principle in this approach includes the electromagnetic field being absorbed by a semiconductor quantum well and the surface plasmon provides a way to focus the light. This method works for the wavelength corresponding to the band edge of the semiconductor materials being used.
A third category of surface plasmon optical modulation is the “Surface Plasmon Amplification” method where light and tunneling electron coupling is mediated by surface plasmon polaritons. The amplification of surface plasmon in the gain region alters the optical intensity.
Implementations of such an approach are described in U.S. Pat. Nos. 7,010,183 and 7,177,515. This approach uses the tunneling junctions to modulate surface plasmons and gratings or prisms to couple between light and surface plasmons. An implementation of this approach requires that the surface plasmons cause tunneling electrons such that additional surface plasmons are produced in a gain region (amplified) by stimulated emission and that the surface plasmons so produced are then directed into the tunneling junction of the diode (wave guiding). This requires a certain waveguide mechanism for surface plasmons to transport from an optical receiving structure to an optical sending structure, thus allowing for a high loss of such surface plasmons propagation.
The prior art approaches for producing surface plasmon optical modulation all possess limitations as discussed above. Therefore, there is a need for a surface plasmon optical modulation design at the nanometer scale that can be fabricated on semiconductor devices using fabrication techniques and equipment readily available in today's nano-fabrication facilities.
The present invention provides such a surface plasmon optical modulation design which is intended to reduce or eliminate the limitations of the foregoing prior art approaches in highly advantageous ways and which provides still further advantages. The present invention improves modulation efficiency (or increases modulation speed) of optical modulators, and does not require that the light wavelength correspond to the band edge of the semiconductor materials used nor does the present invention require amplification of the surface plasmons of the MIS tunneling diode junction. Furthermore, the present invention does not require wave-guiding to transport surface plasmons from an optical receiving structure to an optical sending structure.
2. Related Art
The following U.S. patents are listed as related art to the present invention:
U.S. Pat. No. 4,432,614, filed Dec. 12, 1982, issued Feb. 21, 1984;
U.S. Pat. No. 4,915,482, filed Oct. 27, 1988, issued Apr., 10, 1990;
U.S. Pat. No. 5,067,788, filed Mar. 21, 1990, issued Nov. 26, 1991;
U.S. Pat. No. 5,570,139, filed May 13, 1994, issued Oct. 29, 1996;
U.S. Pat. No. 6,034,809, filed Mar. 26, 1998, issued Mar. 7, 2000;
U.S. Pat. No. 6,611,367, filed Feb. 7, 2000, issued Aug. 26, 2003;
U.S. Pat. No. 7,010,183, filed Mar. 20, 2002, issued Mar. 7, 2006; and
U.S. Pat. No. 7,177,515, filed May 6, 2002, issued Feb. 13, 2007.